Shell tectonics: A mechanical model for strike-slip displacement on Europa

We introduce a new model for producing strike-slip displacement on Europa, which we call shell tectonics. We invoke general principles of stress and failure along faults and include the influence of Europa’s elastic shell when determining the response of faults to periodic tidal stress. We apply a Coulomb failure criterion to determine when and if failure will occur and adopt a linear elastic model for slip and stress release to determine the direction of net offsets along pre-existing faults. Our model reproduces the global-scale strike-slip fault pattern observed on Europa in which left-lateral faults dominate far north of the equator, right-lateral faults do so in the far south, and near-equatorial regions display a mixture of both types of faults. One of the most compelling attributes of the tidal walking model for strike-slip formation on Europa (Hoppa et al., 1999) is its ability to generate this global pattern. The shell tectonics model includes a more physical treatment of fault mechanics than tidal walking and makes a prediction of slip direction along faults by computing the net slip over several orbits. Also, several assumptions made in the tidal walking model are incorporated explicitly in the shell tectonics model. A strike-slip formation model with application to Enceladus has also been proposed (Smith-Konter & Pappalardo, 2008) that includes a mechanical treatment of faults but does not incorporate the effects of the elastic shell. Since this model should be equally applicable to Europa, we present predictions made using this plate-tectonics model along with our shell tectonics predictions. We find that a model neglecting the elastic shell effects does not agree as well with the observations. In addition to global predictions of slip direction, shell tectonics provides an estimate of the relative growth rates of faults and implications for seismicity and heating along faults.